Influence of the pheophytins on the energy conversion processes in photosystem II reaction centres

Abstract

Photosystem II reaction centres (RC) bind two pheophytin a (Pheo) cofactors, but only one functions as electron acceptor [1]. They both contribute to the absorption spectrum at around 680 nm [2], and are at distances of about 10 Å from the nearby chlorophylls (Chl) [3]. It is therefore expected that both Pheos will play a role in electronic interactions with the central Chls of the RC, and should thus be considered in the understanding of energy conversion upon excitation in the long-wavelength part of the QY band. We have examined, by femtosecond pump-probe absorption-difference spectroscopy at 77 K, the effect of replacing the inactive Pheo (RC1x) or both Pheos (RC2x) with 131-hydroxy-131-deoxo-pheophytin a. Excitation was at 683 nm and probe both in the QX and in the QY absorption region. The new pigment absorbs at 654 nm (no 680 -> 654 nm energy transfer takes place at 77 K), and its reduction potential is too negative to allow reduction by singlet-excited P680. Pheo- photoaccumulation is not affected by replacing the inactive Pheo, but it is decreased in parallel with the replacement of the active Pheo in RC2x [2]. The pump-probe results are consistent with these observations. The decreased amplitude of the fast decay phases (in the picosecond timescale) and the larger contribution from excited states to the end spectrum in RC2x are evidence for the absence of a Chl anion intermediate in the primary charge separation reaction. Lifetimes of 1 to 50 ps are assigned to charge separation.